1. Field of the Invention
The present application is directed toward a coating for an exterior surface of a telescope lens, especially a riflescope, a spotting scope, or a binocular to reduce the likelihood of fogging or distortion due to the collection of moisture on the lens without significantly reducing light transmission through the lens in the visible range. More particularly, the present invention is directed toward such coatings that are durable. Still more particularly, such coatings may include two layers with the first layer being an anti-reflective (AR) coating which has either a single layer or multiple layers such that light transmission in the visible range is not significantly reduced and the second coating is a durable hydrophobic coating over the AR coating. Still more particularly, the present invention provides a coating which is not easily scratched or worn away undergoing standard durability and abrasion testing.
2. Description of the Prior Art
Scopes used by sportsmen, military and the like, have improved significantly over the years. However, much of this improvement is lost on cold days or rainy days due to collection of moisture or fogging on the lens which then significantly distorts the image. For example, a hunter on a cold day will often bring the gun to his face to aim through a scope at a target, and in doing so often breathes on the scope's lens. The hunter's breath then fogs the lens. On days when it is raining or drizzling, the moisture from the rain can likewise collect on the lens and distort the image. While the interior face of the lens can be protected against the elements by placement of a moisture free gas within the interior of the scope, the exterior face of the lens is invariably exposed to the elements and incurs the fogging and moisture collection noted above.
Various treatments have been previously utilized that apply hydrophobic materials to various articles, such as sunglasses and automobile windshields, to produce beading of water droplets/or the quick sloughing of moisture, so that the moisture is not retained and spread unevenly on the article due to surface tension so as to produce distortion. However, such treatments reduce the transmissions of light through the article. Typically this reduction in transmission occurs even if the treatment is essentially clear or see through, since the index of refraction of the treatment is different in comparison to the index of refraction of the article causing some amount of reflection to occur. The light that is reflected is then not seen by the viewer which reduces the quality of the image. In articles such as windshields and sunglasses, the amount of reduction in light transmission is not critical, and therefore, a loss of a small amount is not considered to be extremely important.
However, in riflescopes and other telescopic devices, it is extremely important to maintain light transmission in the visible range near 100 percent of incident light in order to ensure that the best possible quality image is seen through the scope. Conventional treatments for rendering articles hydrophobic reduce the transmission of light in certain visible ranges sufficiently to make use of such treatments unacceptable.
Anti-reflective treatments are used on various materials to decrease reflection and increase transmission of light, especially in certain wavelengths. Anti-reflective treatments have been previously used in the prior art for various types of devices, including lenses. Anti-reflective treatments have been especially used where lenses with multiple components are joined together in a side-by-side (or layered) relationship. However, hydrophobic polymers that naturally reduce transmission of light do not adhere to most of the components of anti-reflective treatments that increase transmission of light. Even if the hydrophobic polymer will adhere to an anti-reflective, it has been found through testing that the hydrophobic polymer can often easily be removed by rubbing or just general wear over time.
Consequently, it is desirable to provide an overall coating for an exterior face of a riflescope lens or the like, which includes an externally located hydrophobic polymer that transmits light in the visible range and causes quick beading and sloughing of moisture from the lens. Furthermore, the hydrophobic polymer requires an anti-reflective component that is designed to allow almost one hundred percent of visible light at selected wavelengths to be transmitted through the lens and the hydrophobic polymer. Still furthermore, it is important that the hydrophobic coating be strongly adhered to the lens and not easily removed by rubbing or wear over time. Additionally, what is needed is a hydrophobic lens coating having a certain hardness or durability which provides a certain degree of protection for the lens or any AR layers located on the lens such that the coating is not easily scratched. In this respect, it is desirable that the hydrophobic lens coating be of sufficient hardness and durability to undergo conventional testing without significant damage to the hydrophobic coating, any AR coating, or the lens itself.